You can modify the default message form and add an email address to be filled in to the cc:field by default.

Go to Tools->Forms->Design a form.
Select the 'Message' form.
Highlight the 'cc' field, right click and select properties.
Under the 'Value' tab, set the 'Initial value' to the email address of the other contact.
Save the form.
Everytime you click 'New Message' the cc field will be prefilled with the email address. Note that you can still highlight and delete the email address for messages, so if you want to enforce that, you will also need to mark the field as 'read only' when you modify the properties above. Hope this helps.

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